1. Field of the Invention
Exemplary embodiments of the present invention generally relate to a developing unit containing a two-component developer including magnetic carrier particles and toner particles, a process cartridge including the developing unit, and an image forming apparatus, such as a copier, printer, facsimile machine, and the like, incorporating the developing unit.
2. Discussion of the Related Art
Developing units that develop toner images for electrophotographic printing generally employ either a one-component developer or a two-component developer. While the one-component developer includes toner particles only, the two-component developer includes toner particles and magnetic carrier particles.
Such developing units include a developer bearing member for bearing the developer to convey it to a development region where the developer bearing member faces an image bearing member. The developer bearing member may include a cylindrical development sleeve, for example, constituted as a hollow cylinder the interior of which contains a magnetic field generator capable of generating a magnetic field sufficient to hold the magnetic carrier particles of the developer on the exterior perimeter surface of the development sleeve. Toner particles are then electrostatically attracted to the magnetic carrier particles. As the development sleeve rotates, the toner particles attached to the magnetic carrier particles that are held on the exterior perimeter surface of the development sleeve are conveyed to the development region and then supplied to a latent image formed on a surface of the image bearing member at the development region.
The magnetic field generator has multiple magnetic poles along a direction of rotation of the development sleeve. Examples of such magnetic field generator are a roller-shaped member having magnetic pole-forming parts magnetized by external magnetic fields, a member in which multiple magnets are held by a common holding member so that each of the magnets faces a given direction, and the like.
Developer carried on the exterior perimeter surface of the development sleeve by the magnetic force generated by the magnetic field generator is conveyed in a direction of movement of the surface of the development sleeve as the development sleeve rotates.
FIG. 1 illustrates a schematic configuration of an example of a generally known developing unit 1214, and more specifically an end-on or lateral cross-sectional view thereof. Broken lines in FIG. 1 shows distribution of magnetic flux density (absolute value) in a direction normal to a surface of a developer bearing member. This conventional developing unit 1214 is hereinafter referred to as a first conventional developing unit 1214.
The first conventional developing unit 1214 includes a developer roller 1240 that serves as a developer bearing member and includes an outer development sleeve 1241 serving as a nonmagnetic hollow body and an inner magnetic roller 1247 serving as a magnetic field generator. That is, the developer roller 1240 is formed by the hollow cylindrical development sleeve 1241 made of some non-magnetic material surrounding the magnetic roller 1247, so as to hold developer on an exterior perimeter surface of the development sleeve 1241 by a magnetic force generated by the magnetic roller 1247.
The developing unit 1214 further includes a developer container 1249 for containing developer, screw-shaped agitation/conveyance members 1242 and 1243 for agitating and conveying the developer axially along a direction of a rotary shaft of the development sleeve 1241, and a developer regulating member 1246 for regulating the thickness of a layer of developer carried on the development sleeve 1241.
The developer container 1249 is separated in a first container (i.e., a developer storing chamber) 1249A and a second container (i.e., a developer agitating chamber) 1249B. The first container 1249A is positioned lower than the development sleeve 1241 and extends in an axial direction of the development sleeve 1241. The second container 1249B is disposed adjacent the first container 1249A and also extends in the axial direction of the development sleeve 1241. The first container 1249A includes the agitation/conveyance member 1242 and the second container 249B includes the agitation/conveyance member 1243 that rotates in a direction indicated by arrow “R1” in FIG. 1. The agitation/conveyance member 1243 conveys the developer to a downstream end of the first container 1249A, which corresponds to a far or distal side in FIG. 1. The developer is then conveyed to the second container 1249B through a space or opening where the first container 1249A and the second container 1249B meet and are communicably coupled together. In the second container 1249B, the agitation/conveyance member 1242 conveys the developer to a downstream end of the second container 1249B, which corresponds to a near or proximal side in FIG. 1. Thus, the developer is circulated or recirculated within the developer container 1249.
Toner is generally supplied from a toner bottle, not shown, to the second container 1249B for replenishment, that is, replacing an amount of toner consumed for development. During conveyance of the developer, the magnetic force generated by the magnetic roller 1247 scoops up, or attracts, the developer contained in the first container 1249A, which is then supplied to the development sleeve 1241. Then, the thickness of the layer of thus-supplied developer on the development sleeve 1241 is regulated by the developer regulating member 1246, and the developer passes the development region facing an image bearing member 1012, and returns to the developer container 1249.
The magnetic roller 1247 includes five magnetic poles, which are a magnetic pole S1 for development, a magnetic pole N1 for conveyance, a magnetic pole S2 for developer release at an upstream portion, a magnetic pole S3 for developer release and attraction, and a magnetic pole N2 for regulation. Where the magnetic poles S1, S2, and S3 are implemented as south poles, for example, the magnetic poles N1 and N2 are implemented as north poles, for example.
As the development sleeve 1241 rotates in a direction indicated by arrow “R2” in FIG. 1, the developer held on the development sleeve 1241 is conveyed and then passes by positions facing the magnetic pole S3, the magnetic pole N2, the magnetic pole S1, the magnetic pole N1, and the magnetic pole S2, in this order. After passing the development region, most of the toner particles of the developer are consumed for developing toner images. Therefore, the developer is released or removed from the development sleeve 1241 to return to the developer container 1249 so that new developer can be constantly attracted to the development sleeve 1241. This action is important to provide stable development ability. That is, this action is important to prevent developer carryover or residual retention, in which developer with fewer toner particles remains on the development sleeve 1241 even post-development to be conveyed continuously to the development region again.
When the magnetic pole S2 and the magnetic pole S3 having an identical polarity are disposed adjacent to each other, a developer-releasing region P is formed between the magnetic poles S2 and S3 in the developing unit 1214 shown in FIG. 1 that exerts a release force to cause the developer carried by the development sleeve 1241 to move away from the development sleeve 1241 and toward the first container 1249A of the developer container 1249. That is, the magnetic force generated by the magnetic poles S2 and S3 releases the developer from the development sleeve 1241 in the developer-releasing region P, so that the developer is removed from the development sleeve 1241 and mixed with the developer in the first container 1249A of the developer container 1249.
The first conventional developing unit 1214 shown in FIG. 1 has a polarity inversion point Q on the development sleeve 1241, located within a region extending from the developer-releasing region P to a regulation region where the developer regulating member 1246 regulates the developer attracted to the development sleeve 1241 by the magnetic force generated by the magnetic pole S3. Developer density is high around the polarity inversion point Q because the magnetic force exerted on the developer is relatively strong and a magnetic flux density in a direction normal to the development sleeve 1241 is too small to form a magnetic brush. Accordingly, even if some developer remains on the development sleeve 1241 without being removed therefrom in the developer-releasing region P, such residual developer can be released or scraped off by the high-density developer held in the vicinity of the polarity inversion point Q. For this reason, this conventional developing unit 1214 can effectively prevent developer carryover.
However, such a continuous high-density state of developer in the vicinity of the polarity inversion point Q imposes a constant mechanical stress on the developer particles, causing them to deteriorate. Therefore, an amount of torque to drive the agitation/conveyance member 1243 of the first container 1249A has to be increased and the agitation/conveyance member 1243 has to be more rigid in strength and larger in size, which can lead to an increase both in cost and in size of the first conventional developing unit 1214.
Further, since the developer is subject to a great amount of stress, a speed of progression of implantation of external additives from the toner into the surface of each carrier particle and abrasion of a surface layer film of each carrier particle, both of which are undesirable, may be accelerated. These actions easily can degrade toner chargeability and powder flowability of developer, which in turn can make it difficult to maintain good image quality over an extended period of time. Since the powder properties of developer can degrade easily, an amount of developer conveyed to the development region may decrease especially when the ability of the development sleeve 1241 to convey developer has deteriorated, and good image quality cannot be maintained for an extended period of time.
FIG. 2 illustrates a schematic configuration of another example of a generally known developing unit 1314. This known developing unit 1314 is referred to as a second conventional developing unit 1314. The second conventional developing unit 1314 reduces an amount of stress on the developer. The second conventional developing unit 1314 shown in FIG. 2 is similar to the first conventional developing unit 1214 shown in FIG. 1, except that a single magnetic pole capable of performing removal, attraction, and regulation of developer simultaneously is provided in the vicinity of a developer regulating member 1346, instead of the known magnetic poles S3 and N2 shown in FIG. 1.
Similar to the first conventional developing unit 1214, the second conventional developing unit 1314 includes a developer roller 1340 that serves as a developer bearing member and is disposed facing the image bearing member 1012, and includes an outer development sleeve 1341 serving as a nonmagnetic hollow body and an inner magnetic roller 1347 serving as a magnetic field generator. The developing unit 1314 further includes a developer container 1349 for containing developer, screw-shaped agitation/conveyance members 1342 and 1343, and the developer regulating member 1346 for regulating the thickness of a layer of developer carried on the development sleeve 1341 that rotates in a direction indicated by arrow “R2” in FIG. 2. The developer container 1349 is separated into a first container (i.e., a developer storing chamber) 1349A and a second container (i.e., a developer agitating chamber) 1349B.
According to the second conventional developing unit 1314 shown in FIG. 2, the developer that cannot be attracted by the magnetic force of the magnetic pole N3 may fall to the agitation/conveyance screw 1343 (which rotates in a direction indicated by arrow “R1” in FIG. 2) in a region upstream from the regulation region where the developer regulating member 1346 regulates the thickness of a layer of developer in a direction of conveyance of developer by the development sleeve 1341 of the developing roller 1340. (Hereinafter, “upstream” and “downstream” indicate an upstream side and downstream side from a given specific position in a direction of conveyance of developer by the development sleeve 1341, respectively.) Such an arrangement prevents a large body of developer from accumulating in the region, thereby reducing the stress on the developer.
Further, yet another example of a known developing unit in which the above-described magnetic pole N3 is disposed adjacent a developer regulating member is disclosed. This known developing unit also can reduce the amount of stress on the developer and for the same reasons as described above. This known developing unit is referred to as a third conventional developing unit.
Further, yet another example of a known developing unit includes a developer regulating member and a cooling unit. This known developing unit is referred to as a fourth conventional developing unit. The developer regulating member of the fourth conventional developing unit is a hollow metallic member, extending in a direction perpendicular to a direction of movement of the surface of the developer bearing member. The cooling unit cools the developer regulating member from the interior of the hollow member so as to reduce an increase in temperature of developer in the vicinity of the developer regulating member.
However, a problem arises in the second conventional developing unit 1341 and the third conventional developing units. In these conventional developing units, a decrease in accumulation of developer in a region upstream from the regulation region where the developer regulating member 1346 is disposed can and has caused uneven image density, a matter on which the present inventors have conducted extensive research to determine why the accumulated developer causes such unevenness in image density.
Specifically, after image development is performed in the second conventional developing unit 1314 and the third conventional developing unit, the developer on the development sleeve 1341 is removed therefrom at the developer-releasing region P. The developer released at the developer-releasing region P then falls onto other developer contained in a first container or developer storing chamber 1349A of a developer container 1349, and is then conveyed parallel to the development sleeve 1341 in an axial direction of the development sleeve 1341 in the first container 1349A while the agitation/conveyance screw 1343 agitates both the pre- and post-development developers.
The post-development developer can be attracted by the magnetic force generated by the magnetic pole N3 immediately after falling onto the stored developer in the first container 1349A. Since the post-development developer that has just fallen from the development sleeve 1341 is not sufficiently mixed and agitated with the stored developer, any developer attracted thereafter may include both developer with a low toner density as well as developer with a high toner density when conveyed to the regulation region.
Different from the second conventional developing unit 1314 shown in FIG. 2, in the first conventional developing unit shown in FIG. 1, high-density developer accumulates in a region in the vicinity of the polarity inversion point Q, which is located upstream from the regulation region. The attracted developer is then mixed with the accumulated developer, now highly stressed, when passing the region of developer accumulation. Therefore, even if the post-development developer is scooped up when not mixed sufficiently with other developer, any difference in toner densities of the developers may be erased in the region where the developer accumulates, and therefore unevenness in image density is not likely to occur or occurs less often.
However, since developer does not accumulate in the region located on the upstream side of the regulation region in the configuration shown in FIG. 2, which corresponds to the second and third conventional developing units, differences in toner density of the developers may not be erased even in the developer accumulation region. Therefore, if the post-development developer is scooped up immediately after falling onto the stored developer in the first container 1349A of a developer container 1349, the scooped developer includes both developer with a low toner density as well as developer with a high toner density as discrete, separate streams or portions when conveyed to the development region via the regulation region, which can easily cause fluctuations in output image density.